NAME

DESCRIPTION

Data: Numbers

Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?

Internally, your computer represents floating-point numbers in binary.
Digital (as in powers of two) computers cannot store all numbers
exactly. Some real numbers lose precision in the process. This is a
problem with how computers store numbers and affects all computer
languages, not just Perl.

perlnumber shows the gory details of number representations and
conversions.

will in most computers print 0, not 1, because even such simple
numbers as 0.6 and 0.2 cannot be presented exactly by floating-point
numbers. What you think in the above as 'three' is really more like
2.9999999999999995559.

Why isn't my octal data interpreted correctly?

(contributed by brian d foy)

You're probably trying to convert a string to a number, which Perl only
converts as a decimal number. When Perl converts a string to a number, it
ignores leading spaces and zeroes, then assumes the rest of the digits
are in base 10:

This problem usually involves one of the Perl built-ins that has the
same name a Unix command that uses octal numbers as arguments on the
command line. In this example, chmod on the command line knows that
its first argument is octal because that's what it does:

%prompt> chmod 644 file

If you want to use the same literal digits (644) in Perl, you have to tell
Perl to treat them as octal numbers either by prefixing the digits with
a 0
or using oct:

In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
module. With 5.004, the Math::Trig
module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it
uses the Math::Complex
module and some functions can break out from
the real axis into the complex plane, for example the inverse sine of
2.

Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely. In these
cases, it probably pays not to trust whichever system rounding is
being used by Perl, but to instead implement the rounding function you
need yourself.

To see why, notice how you'll still have an issue on half-way-point
alternation:

for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}

0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7

0.8 0.8 0.9 0.9 1.0 1.0

Don't blame Perl. It's the same as in C. IEEE says we have to do
this. Perl numbers whose absolute values are integers under 2**31 (on
32 bit machines) will work pretty much like mathematical integers.
Other numbers are not guaranteed.

How do I convert between numeric representations/bases/radixes?

As always with Perl there is more than one way to do it. Below are a
few examples of approaches to making common conversions between number
representations. This is intended to be representational rather than
exhaustive.

Some of the examples later in perlfaq4 use the Bit::Vector
module from CPAN. The reason you might choose Bit::Vector
over the
perl built in functions is that it works with numbers of ANY size,
that it is optimized for speed on some operations, and for at least
some programmers the notation might be familiar.

The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
are left as an exercise to the inclined reader.

Why doesn't & work the way I want it to?

The behavior of binary arithmetic operators depends on whether they're
used on numbers or strings. The operators treat a string as a series
of bits and work with that (the string "3"
is the bit pattern
00110011
). The operators work with the binary form of a number
(the number 3
is treated as the bit pattern 00000011
).

Why aren't my random numbers random?

5.004 and later automatically call srand at the beginning. Don't
call srand more than once--you make your numbers less random,
rather than more.

Computers are good at being predictable and bad at being random
(despite appearances caused by bugs in your programs :-). see the
random article in the "Far More Than You Ever Wanted To Know"
collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy
of Tom Phoenix, talks more about this. John von Neumann said, "Anyone
who attempts to generate random numbers by deterministic means is, of
course, living in a state of sin."

If you want numbers that are more random than rand with srand
provides, you should also check out the Math::TrulyRandom
module from
CPAN. It uses the imperfections in your system's timer to generate
random numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating system, look at
"Numerical Recipes in C" at http://www.nr.com/ .

How do I get a random number between X and Y?

To get a random number between two values, you can use the rand()
built-in to get a random number between 0 and 1. From there, you shift
that into the range that you want.

rand($x) returns a number such that 0 <= rand($x) < $x
. Thus
what you want to have perl figure out is a random number in the range
from 0 to the difference between your X and Y.

That is, to get a number between 10 and 15, inclusive, you want a
random number between 0 and 5 that you can then add to 10.

How do I find the current century or millennium?

On some systems, the POSIX
module's strftime()
function has been
extended in a non-standard way to use a %C
format, which they
sometimes claim is the "century". It isn't, because on most such
systems, this is only the first two digits of the four-digit year, and
thus cannot be used to reliably determine the current century or
millennium.

How can I compare two dates and find the difference?

(contributed by brian d foy)

You could just store all your dates as a number and then subtract.
Life isn't always that simple though. If you want to work with
formatted dates, the Date::Manip
, Date::Calc
, or DateTime
modules can help you.

How can I take a string and turn it into epoch seconds?

If it's a regular enough string that it always has the same format,
you can split it up and pass the parts to timelocal
in the standard
Time::Local
module. Otherwise, you should look into the Date::Calc
and Date::Manip
modules from CPAN.

How can I find the Julian Day?

(contributed by brian d foy and Dave Cross)

You can use the Time::JulianDay
module available on CPAN. Ensure
that you really want to find a Julian day, though, as many people have
different ideas about Julian days. See
http://www.hermetic.ch/cal_stud/jdn.htm for instance.

You can also try the DateTime
module, which can convert a date/time
to a Julian Day.

$ perl -MDateTime -le'print DateTime->today->jd'

2453401.5

Or the modified Julian Day

$ perl -MDateTime -le'print DateTime->today->mjd'

53401

Or even the day of the year (which is what some people think of as a
Julian day)

$ perl -MDateTime -le'print DateTime->today->doy'

31

How do I find yesterday's date?

(contributed by brian d foy)

Use one of the Date modules. The DateTime
module makes it simple, and
give you the same time of day, only the day before.

Most people try to use the time rather than the calendar to figure out
dates, but that assumes that days are twenty-four hours each. For
most people, there are two days a year when they aren't: the switch to
and from summer time throws this off. Let the modules do the work.

If you absolutely must do it yourself (or can't use one of the
modules), here's a solution using Time::Local
, which comes with
Perl:

In this case, you measure the day starting at noon, and subtract 24
hours. Even if the length of the calendar day is 23 or 25 hours,
you'll still end up on the previous calendar day, although not at
noon. Since you don't care about the time, the one hour difference
doesn't matter and you end up with the previous date.

Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?

(contributed by brian d foy)

Perl itself never had a Y2K problem, although that never stopped people
from creating Y2K problems on their own. See the documentation for
localtime for its proper use.

Starting with Perl 5.11, localtime and gmtime can handle dates past
03:14:08 January 19, 2038, when a 32-bit based time would overflow. You
still might get a warning on a 32-bit perl
:

% perl5.11.2 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )'

Integer overflow in hexadecimal number at -e line 1.

Wed Nov 1 19:42:39 5576711

On a 64-bit perl
, you can get even larger dates for those really long
running projects:

% perl5.11.2 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'

Thu Nov 2 00:42:39 5576711

You're still out of luck if you need to keep tracking of decaying protons
though.

Data: Strings

How do I validate input?

(contributed by brian d foy)

There are many ways to ensure that values are what you expect or
want to accept. Besides the specific examples that we cover in the
perlfaq, you can also look at the modules with "Assert" and "Validate"
in their names, along with other modules such as Regexp::Common
.

Some modules have validation for particular types of input, such
as Business::ISBN
, Business::CreditCard
, Email::Valid
,
and Data::Validate::IP
.

How do I unescape a string?

It depends just what you mean by "escape". URL escapes are dealt
with in perlfaq9. Shell escapes with the backslash (\
)
character are removed with

s/\\(.)/$1/g;

This won't expand "\n"
or "\t"
or any other special escapes.

How do I remove consecutive pairs of characters?

(contributed by brian d foy)

You can use the substitution operator to find pairs of characters (or
runs of characters) and replace them with a single instance. In this
substitution, we find a character in (.). The memory parentheses
store the matched character in the back-reference \1
and we use
that to require that the same thing immediately follow it. We replace
that part of the string with the character in $1
.

s/(.)\1/$1/g;

We can also use the transliteration operator, tr///. In this
example, the search list side of our tr/// contains nothing, but
the c
option complements that so it contains everything. The
replacement list also contains nothing, so the transliteration is
almost a no-op since it won't do any replacements (or more exactly,
replace the character with itself). However, the s option squashes
duplicated and consecutive characters in the string so a character
does not show up next to itself

How do I expand function calls in a string?

(contributed by brian d foy)

This is documented in perlref, and although it's not the easiest
thing to read, it does work. In each of these examples, we call the
function inside the braces used to dereference a reference. If we
have more than one return value, we can construct and dereference an
anonymous array. In this case, we call the function in list context.

If we want to call the function in scalar context, we have to do a bit
more work. We can really have any code we like inside the braces, so
we simply have to end with the scalar reference, although how you do
that is up to you, and you can use code inside the braces. Note that
the use of parens creates a list context, so we need scalar to
force the scalar context on the function:

The Interpolation
module can also do a lot of magic for you. You can
specify a variable name, in this case E
, to set up a tied hash that
does the interpolation for you. It has several other methods to do this
as well.

How do I find matching/nesting anything?

This isn't something that can be done in one regular expression, no
matter how complicated. To find something between two single
characters, a pattern like /x([^x]*)x/
will get the intervening
bits in $1. For multiple ones, then something more like
/alpha(.*?)omega/
would be needed. But none of these deals with
nested patterns. For balanced expressions using (, {, [ or
<
as delimiters, use the CPAN module Regexp::Common, or see
(??{ code }) in perlre. For other cases, you'll have to write a
parser.

If you are serious about writing a parser, there are a number of
modules or oddities that will make your life a lot easier. There are
the CPAN modules Parse::RecDescent
, Parse::Yapp
, and
Text::Balanced
; and the byacc
program. Starting from perl 5.8
the Text::Balanced
is part of the standard distribution.

One simple destructive, inside-out approach that you might try is to
pull out the smallest nesting parts one at a time:

while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs){

# do something with $1

}

A more complicated and sneaky approach is to make Perl's regular
expression engine do it for you. This is courtesy Dean Inada, and
rather has the nature of an Obfuscated Perl Contest entry, but it
really does work:

How do I change the Nth occurrence of something?

You have to keep track of N yourself. For example, let's say you want
to change the fifth occurrence of "whoever"
or "whomever"
into
"whosoever"
or "whomsoever"
, case insensitively. These
all assume that $_ contains the string to be altered.

$count = 0;

s{((whom?)ever)}{

++$count == 5 # is it the 5th?

? "${2}soever" # yes, swap

: $1 # renege and leave it there

}ige;

In the more general case, you can use the /g modifier in a while
loop, keeping count of matches.

This is fine if you are just looking for a single character. However,
if you are trying to count multiple character substrings within a
larger string, tr/// won't work. What you can do is wrap a while()
loop around a global pattern match. For example, let's count negative
integers:

It's not as easy a problem as it looks. How many words do you think
are in there? Wait for it... wait for it.... If you answered 5
you're right. Perl words are groups of \w+
, but that's not what
you want to capitalize. How is Perl supposed to know not to capitalize
that s after the apostrophe? You could try a regular expression:

$string =~ s/ (

(^\w) #at the beginning of the line

| # or

(\s\w) #preceded by whitespace

)

/\U$1/xg;

$string =~ s/([\w']+)/\u\L$1/g;

Now, what if you don't want to capitalize that "and"? Just use
Text::Autoformat and get on with the next problem. :)

How can I split a [character] delimited string except when inside [character]?

Several modules can handle this sort of parsing--Text::Balanced
,
Text::CSV
, Text::CSV_XS
, and Text::ParseWords
, among others.

Take the example case of trying to split a string that is
comma-separated into its different fields. You can't use split(/,/)
because you shouldn't split if the comma is inside quotes. For
example, take a data line like this:

Due to the restriction of the quotes, this is a fairly complex
problem. Thankfully, we have Jeffrey Friedl, author of
Mastering Regular Expressions, to handle these for us. He
suggests (assuming your string is contained in $text
):

How do I strip blank space from the beginning/end of a string?

A substitution can do this for you. For a single line, you want to
replace all the leading or trailing whitespace with nothing. You
can do that with a pair of substitutions.

s/^\s+//;

s/\s+$//;

You can also write that as a single substitution, although it turns
out the combined statement is slower than the separate ones. That
might not matter to you, though.

s/^\s+|\s+$//g;

In this regular expression, the alternation matches either at the
beginning or the end of the string since the anchors have a lower
precedence than the alternation. With the /g flag, the substitution
makes all possible matches, so it gets both. Remember, the trailing
newline matches the \s+, and the $
anchor can match to the
physical end of the string, so the newline disappears too. Just add
the newline to the output, which has the added benefit of preserving
"blank" (consisting entirely of whitespace) lines which the ^\s+
would remove all by itself.

For a multi-line string, you can apply the regular expression
to each logical line in the string by adding the /m flag (for
"multi-line"). With the /m flag, the $
matches before an
embedded newline, so it doesn't remove it. It still removes the
newline at the end of the string.

$string =~ s/^\s+|\s+$//gm;

Remember that lines consisting entirely of whitespace will disappear,
since the first part of the alternation can match the entire string
and replace it with nothing. If need to keep embedded blank lines,
you have to do a little more work. Instead of matching any whitespace
(since that includes a newline), just match the other whitespace.

$string =~ s/^[\t\f ]+|[\t\f ]+$//mg;

How do I pad a string with blanks or pad a number with zeroes?

In the following examples, $pad_len
is the length to which you wish
to pad the string, $text
or $num
contains the string to be padded,
and $pad_char
contains the padding character. You can use a single
character string constant instead of the $pad_char
variable if you
know what it is in advance. And in the same way you can use an integer in
place of $pad_len
if you know the pad length in advance.

The simplest method uses the sprintf function. It can pad on the left
or right with blanks and on the left with zeroes and it will not
truncate the result. The pack function can only pad strings on the
right with blanks and it will truncate the result to a maximum length of
$pad_len
.

If you need to pad with a character other than blank or zero you can use
one of the following methods. They all generate a pad string with the
x
operator and combine that with $text
. These methods do
not truncate $text
.

If you want to work with comma-separated values, don't do this since
that format is a bit more complicated. Use one of the modules that
handle that format, such as Text::CSV
, Text::CSV_XS
, or
Text::CSV_PP
.

If you want to break apart an entire line of fixed columns, you can use
unpack with the A (ASCII) format. By using a number after the format
specifier, you can denote the column width. See the pack and unpack
entries in perlfunc for more details.

However, for the one-off simple case where I don't want to pull out a
full templating system, I'll use a string that has two Perl scalar
variables in it. In this example, I want to expand $foo
and $bar
to their variable's values:

One way I can do this involves the substitution operator and a double
/e flag. The first /e evaluates $1
on the replacement side and
turns it into $foo
. The second /e starts with $foo
and replaces
it with its value. $foo
, then, turns into 'Fred', and that's finally
what's left in the string:

$string =~ s/(\$\w+)/$1/eeg;# 'Say hello to Fred and Barney'

The /e will also silently ignore violations of strict, replacing
undefined variable names with the empty string. Since I'm using the
/e flag (twice even!), I have all of the same security problems I
have with eval in its string form. If there's something odd in
$foo
, perhaps something like @{[system"rm -rf /"]}
, then
I could get myself in trouble.

To get around the security problem, I could also pull the values from
a hash instead of evaluating variable names. Using a single /e, I
can check the hash to ensure the value exists, and if it doesn't, I
can replace the missing value with a marker, in this case ??? to
signal that I missed something:

What's wrong with always quoting "$vars"?

The problem is that those double-quotes force
stringification--coercing numbers and references into strings--even
when you don't want them to be strings. Think of it this way:
double-quote expansion is used to produce new strings. If you already
have a string, why do you need more?

You can also get into subtle problems on those few operations in Perl
that actually do care about the difference between a string and a
number, such as the magical ++
autoincrement operator or the
syscall() function.

But the HERE_TARGET must still be flush against the margin.
If you want that indented also, you'll have to quote
in the indentation.

($quote = <<' FINIS') =~ s/^\s+//gm;

...we will have peace, when you and all your works have

perished--and the works of your dark master to whom you

would deliver us. You are a liar, Saruman, and a corrupter

of men's hearts. --Theoden in /usr/src/perl/taint.c

FINIS

$quote =~ s/\s+--/\n--/;

A nice general-purpose fixer-upper function for indented here documents
follows. It expects to be called with a here document as its argument.
It looks to see whether each line begins with a common substring, and
if so, strips that substring off. Otherwise, it takes the amount of leading
whitespace found on the first line and removes that much off each
subsequent line.

Or with a fixed amount of leading whitespace, with remaining
indentation correctly preserved:

$poem = fix<<EVER_ON_AND_ON;

Now far ahead the Road has gone,

And I must follow, if I can,

Pursuing it with eager feet,

Until it joins some larger way

Where many paths and errands meet.

And whither then? I cannot say.

--Bilbo in /usr/src/perl/pp_ctl.c

EVER_ON_AND_ON

Data: Arrays

What is the difference between a list and an array?

(contributed by brian d foy)

A list is a fixed collection of scalars. An array is a variable that
holds a variable collection of scalars. An array can supply its collection
for list operations, so list operations also work on arrays:

Array operations, which change the scalars, reaaranges them, or adds
or subtracts some scalars, only work on arrays. These can't work on a
list, which is fixed. Array operations include shift, unshift,
push, pop, and splice.

An array can also change its length:

$#animals = 1;# truncate to two elements

$#animals = 10000;# pre-extend to 10,001 elements

You can change an array element, but you can't change a list element:

$animals[0] = 'Rottweiler';

qw( dog cat bird )[0] = 'Rottweiler';# syntax error!

foreach (@animals){

s/^d/fr/;# works fine

}

foreach (qw( dog cat bird )){

s/^d/fr/;# Error! Modification of read only value!

}

However, if the list element is itself a variable, it appears that you
can change a list element. However, the list element is the variable, not
the data. You're not changing the list element, but something the list
element refers to. The list element itself doesn't change: it's still
the same variable.

You also have to be careful about context. You can assign an array to
a scalar to get the number of elements in the array. This only works
for arrays, though:

If you try to do the same thing with what you think is a list, you
get a quite different result. Although it looks like you have a list
on the righthand side, Perl actually sees a bunch of scalars separated
by a comma:

Since you're assigning to a scalar, the righthand side is in scalar
context. The comma operator (yes, it's an operator!) in scalar
context evaluates its lefthand side, throws away the result, and
evaluates it's righthand side and returns the result. In effect,
that list-lookalike assigns to $scalar
it's rightmost value. Many
people mess this up becuase they choose a list-lookalike whose
last element is also the count they expect:

What is the difference between $array[1] and @array[1]?

(contributed by brian d foy)

The difference is the sigil, that special character in front of the
array name. The $
sigil means "exactly one item", while the @
sigil means "zero or more items". The $
gets you a single scalar,
while the @
gets you a list.

The confusion arises because people incorrectly assume that the sigil
denotes the variable type.

The $array[1]
is a single-element access to the array. It's going
to return the item in index 1 (or undef if there is no item there).
If you intend to get exactly one element from the array, this is the
form you should use.

The @array[1]
is an array slice, although it has only one index.
You can pull out multiple elements simultaneously by specifying
additional indices as a list, like @array[1,4,3,0]
.

Using a slice on the lefthand side of the assignment supplies list
context to the righthand side. This can lead to unexpected results.
For instance, if you want to read a single line from a filehandle,
assigning to a scalar value is fine:

$array[1] = <STDIN>;

However, in list context, the line input operator returns all of the
lines as a list. The first line goes into @array[1]
and the rest
of the lines mysteriously disappear:

@array[1] = <STDIN>;# most likely not what you want

Either the usewarnings
pragma or the -w flag will warn you when
you use an array slice with a single index.

How can I remove duplicate elements from a list or array?

(contributed by brian d foy)

Use a hash. When you think the words "unique" or "duplicated", think
"hash keys".

If you don't care about the order of the elements, you could just
create the hash then extract the keys. It's not important how you
create that hash: just that you use keys to get the unique
elements.

If you want to use a module, try the uniq
function from
List::MoreUtils
. In list context it returns the unique elements,
preserving their order in the list. In scalar context, it returns the
number of unique elements.

You can also go through each element and skip the ones you've seen
before. Use a hash to keep track. The first time the loop sees an
element, that element has no key in %Seen
. The next statement
creates the key and immediately uses its value, which is undef, so
the loop continues to the push and increments the value for that
key. The next time the loop sees that same element, its key exists in
the hash and the value for that key is true (since it's not 0 or
undef), so the next skips that iteration and the loop goes to the
next element.

How can I tell whether a certain element is contained in a list or array?

(portions of this answer contributed by Anno Siegel and brian d foy)

Hearing the word "in" is an indication that you probably should have
used a hash, not a list or array, to store your data. Hashes are
designed to answer this question quickly and efficiently. Arrays aren't.

That being said, there are several ways to approach this. In Perl 5.10
and later, you can use the smart match operator to check that an item is
contained in an array or a hash:

With earlier versions of Perl, you have to do a bit more work. If you
are going to make this query many times over arbitrary string values,
the fastest way is probably to invert the original array and maintain a
hash whose keys are the first array's values:

@blues = qw/azure cerulean teal turquoise lapis-lazuli/;

%is_blue = ();

for (@blues){$is_blue{$_} = 1}

Now you can check whether $is_blue{$some_color}
. It might have
been a good idea to keep the blues all in a hash in the first place.

If the values are all small integers, you could use a simple indexed
array. This kind of an array will take up less space:

@primes = (2,3,5,7,11,13,17,19,23,29,31);

@is_tiny_prime = ();

for (@primes){$is_tiny_prime[$_] = 1}

# or simply @istiny_prime[@primes] = (1) x @primes;

Now you check whether $is_tiny_prime[$some_number].

If the values in question are integers instead of strings, you can save
quite a lot of space by using bit strings instead:

These methods guarantee fast individual tests but require a re-organization
of the original list or array. They only pay off if you have to test
multiple values against the same array.

If you are testing only once, the standard module List::Util
exports
the function first
for this purpose. It works by stopping once it
finds the element. It's written in C for speed, and its Perl equivalent
looks like this subroutine:

If speed is of little concern, the common idiom uses grep in scalar context
(which returns the number of items that passed its condition) to traverse the
entire list. This does have the benefit of telling you how many matches it
found, though.

The first reports that both those the hashes contain the same data,
while the second reports that they do not. Which you prefer is left as
an exercise to the reader.

How do I find the first array element for which a condition is true?

To find the first array element which satisfies a condition, you can
use the first()
function in the List::Util
module, which comes
with Perl 5.8. This example finds the first element that contains
"Perl".

How do I handle linked lists?

In general, you usually don't need a linked list in Perl, since with
regular arrays, you can push and pop or shift and unshift at either
end, or you can use splice to add and/or remove arbitrary number of
elements at arbitrary points. Both pop and shift are O(1)
operations on Perl's dynamic arrays. In the absence of shifts and
pops, push in general needs to reallocate on the order every log(N)
times, and unshift will need to copy pointers each time.

If you really, really wanted, you could use structures as described in
perldsc or perltoot and do just what the algorithm book tells
you to do. For example, imagine a list node like this:

This is bad because splice is already O(N), and since you do it N
times, you just invented a quadratic algorithm; that is, O(N**2).
This does not scale, although Perl is so efficient that you probably
won't notice this until you have rather largish arrays.

How do I process/modify each element of an array?

Use for
/foreach
:

for (@lines){

s/foo/bar/;# change that word

tr/XZ/ZX/;# swap those letters

}

Here's another; let's compute spherical volumes:

for (@volumes = @radii){# @volumes has changed parts

$_ **= 3;

$_ *= (4/3) * 3.14159;# this will be constant folded

}

which can also be done with map() which is made to transform
one list into another:

If you want to do the same thing to modify the values of the
hash, you can use the values function. As of Perl 5.6
the values are not copied, so if you modify $orbit (in this
case), you modify the value.

Here's a little program that generates all permutations of all the
words on each line of input. The algorithm embodied in the
permute()
function is discussed in Volume 4 (still unpublished) of
Knuth's The Art of Computer Programming and will work on any list:

The Algorithm::Loops
module also provides the NextPermute
and
NextPermuteNum
functions which efficiently find all unique permutations
of an array, even if it contains duplicate values, modifying it in-place:
if its elements are in reverse-sorted order then the array is reversed,
making it sorted, and it returns false; otherwise the next
permutation is returned.

NextPermute
uses string order and NextPermuteNum
numeric order, so
you can enumerate all the permutations of 0..9
like this:

How do I sort an array by (anything)?

The default sort function is cmp, string comparison, which would
sort (1,2,10)
into (1,10,2)
. <=>
, used above, is
the numerical comparison operator.

If you have a complicated function needed to pull out the part you
want to sort on, then don't do it inside the sort function. Pull it
out first, because the sort BLOCK can be called many times for the
same element. Here's an example of how to pull out the first word
after the first number on each item, and then sort those words
case-insensitively.

How do I manipulate arrays of bits?

For example, you don't have to store individual bits in an array
(which would mean that you're wasting a lot of space). To convert an
array of bits to a string, use vec() to set the right bits. This
sets $vec
to have bit N set only if $ints[N]
was set:

Why does defined() return true on empty arrays and hashes?

The short story is that you should probably only use defined on scalars or
functions, not on aggregates (arrays and hashes). See defined
in the 5.004 release or later of Perl for more detail.

Data: Hashes (Associative Arrays)

How do I process an entire hash?

(contributed by brian d foy)

There are a couple of ways that you can process an entire hash. You
can get a list of keys, then go through each key, or grab a one
key-value pair at a time.

To go through all of the keys, use the keys function. This extracts
all of the keys of the hash and gives them back to you as a list. You
can then get the value through the particular key you're processing:

The each operator returns the pairs in apparently random order, so if
ordering matters to you, you'll have to stick with the keys method.

The each() operator can be a bit tricky though. You can't add or
delete keys of the hash while you're using it without possibly
skipping or re-processing some pairs after Perl internally rehashes
all of the elements. Additionally, a hash has only one iterator, so if
you use keys, values, or each on the same hash, you can reset
the iterator and mess up your processing. See the each entry in
perlfunc for more details.

How do I merge two hashes?

(contributed by brian d foy)

Before you decide to merge two hashes, you have to decide what to do
if both hashes contain keys that are the same and if you want to leave
the original hashes as they were.

If you want to preserve the original hashes, copy one hash (%hash1
)
to a new hash (%new_hash
), then add the keys from the other hash
(%hash2
to the new hash. Checking that the key already exists in
%new_hash
gives you a chance to decide what to do with the
duplicates:

If you don't care that one hash overwrites keys and values from the other, you
could just use a hash slice to add one hash to another. In this case, values
from %hash2
replace values from %hash1
when they have keys in common:

What happens if I add or remove keys from a hash while iterating over it?

(contributed by brian d foy)

The easy answer is "Don't do that!"

If you iterate through the hash with each(), you can delete the key
most recently returned without worrying about it. If you delete or add
other keys, the iterator may skip or double up on them since perl
may rearrange the hash table. See the
entry for each() in perlfunc.

If your hash could have repeated values, the methods above will only find
one of the associated keys. This may or may not worry you. If it does
worry you, you can always reverse the hash into a hash of arrays instead:

The keys() function also resets the iterator, which means that you may
see strange results if you use this between uses of other hash operators
such as each().

How do I sort a hash (optionally by value instead of key)?

(contributed by brian d foy)

To sort a hash, start with the keys. In this example, we give the list of
keys to the sort function which then compares them ASCIIbetically (which
might be affected by your locale settings). The output list has the keys
in ASCIIbetical order. Once we have the keys, we can go through them to
create a report which lists the keys in ASCIIbetical order.

We could get more fancy in the sort() block though. Instead of
comparing the keys, we can compute a value with them and use that
value as the comparison.

For instance, to make our report order case-insensitive, we use
the \L
sequence in a double-quoted string to make everything
lowercase. The sort() block then compares the lowercased
values to determine in which order to put the keys.

How can I always keep my hash sorted?

You can look into using the DB_File
module and tie() using the
$DB_BTREE
hash bindings as documented in In Memory Databases in DB_File. The Tie::IxHash
module from CPAN might also be
instructive. Although this does keep your hash sorted, you might not
like the slow down you suffer from the tie interface. Are you sure you
need to do this? :)

What's the difference between "delete" and "undef" with hashes?

Hashes contain pairs of scalars: the first is the key, the
second is the value. The key will be coerced to a string,
although the value can be any kind of scalar: string,
number, or reference. If a key $key
is present in
%hash, exists($hash{$key}) will return true. The value
for a given key can be undef, in which case
$hash{$key}
will be undef while exists$hash{$key}
will return true. This corresponds to ($key
, undef)
being in the hash.

Why don't my tied hashes make the defined/exists distinction?

This depends on the tied hash's implementation of EXISTS().
For example, there isn't the concept of undef with hashes
that are tied to DBM* files. It also means that exists() and
defined() do the same thing with a DBM* file, and what they
end up doing is not what they do with ordinary hashes.

How do I reset an each() operation part-way through?

(contributed by brian d foy)

You can use the keys or values functions to reset each. To
simply reset the iterator used by each without doing anything else,
use one of them in void context:

Passing $hash{ 'foo' }
to a subroutine used to be a special case, though.
Since you could assign directly to $_[0]
, Perl had to be ready to
make that assignment so it created the hash key ahead of time:

However, if you want the old behavior (and think carefully about that
because it's a weird side effect), you can pass a hash slice instead.
Perl 5.004 didn't make this a special case:

my_sub(@hash{ qw/foo/ } );

How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?

Usually a hash ref, perhaps like this:

$record = {

NAME=>"Jason",

EMPNO=>132,

TITLE=>"deputy peon",

AGE=>23,

SALARY=>37_000,

PALS=>["Norbert","Rhys","Phineas"],

};

References are documented in perlref and the upcoming perlreftut.
Examples of complex data structures are given in perldsc and
perllol. Examples of structures and object-oriented classes are
in perltoot.

How can I use a reference as a hash key?

(contributed by brian d foy and Ben Morrow)

Hash keys are strings, so you can't really use a reference as the key.
When you try to do that, perl turns the reference into its stringified
form (for instance, HASH(0xDEADBEEF)
). From there you can't get
back the reference from the stringified form, at least without doing
some extra work on your own.

Remember that the entry in the hash will still be there even if
the referenced variable goes out of scope, and that it is entirely
possible for Perl to subsequently allocate a different variable at
the same address. This will mean a new variable might accidentally
be associated with the value for an old.

If you have Perl 5.10 or later, and you just want to store a value
against the reference for lookup later, you can use the core
Hash::Util::Fieldhash module. This will also handle renaming the
keys if you use multiple threads (which causes all variables to be
reallocated at new addresses, changing their stringification), and
garbage-collecting the entries when the referenced variable goes out
of scope.

If you actually need to be able to get a real reference back from
each hash entry, you can use the Tie::RefHash module, which does the
required work for you.

Data: Misc

How do I handle binary data correctly?

Perl is binary clean, so it can handle binary data just fine.
On Windows or DOS, however, you have to use binmode for binary
files to avoid conversions for line endings. In general, you should
use binmode any time you want to work with binary data.

There are also some commonly used modules for the task.
Scalar::Util (distributed with 5.8) provides access to perl's
internal function looks_like_number
for determining whether a
variable looks like a number. Data::Types exports functions that
validate data types using both the above and other regular
expressions. Thirdly, there is Regexp::Common
which has regular
expressions to match various types of numbers. Those three modules are
available from the CPAN.

If you're on a POSIX system, Perl supports the POSIX::strtod
function. Its semantics are somewhat cumbersome, so here's a
getnum
wrapper function for more convenient access. This function
takes a string and returns the number it found, or undef for input
that isn't a C float. The is_numeric
function is a front end to
getnum
if you just want to say, "Is this a float?"

Or you could check out the String::Scanf module on the CPAN
instead. The POSIX
module (part of the standard Perl distribution)
provides the strtod
and strtol
for converting strings to double
and longs, respectively.

How do I keep persistent data across program calls?

For some specific applications, you can use one of the DBM modules.
See AnyDBM_File. More generically, you should consult the FreezeThaw
or Storable
modules from CPAN. Starting from Perl 5.8 Storable
is part
of the standard distribution. Here's one example using Storable
's store
and retrieve
functions:

How do I print out or copy a recursive data structure?

The Data::Dumper
module on CPAN (or the 5.005 release of Perl) is great
for printing out data structures. The Storable
module on CPAN (or the
5.8 release of Perl), provides a function called dclone
that recursively
copies its argument.

Where $r1
can be a reference to any kind of data structure you'd like.
It will be deeply copied. Because dclone
takes and returns references,
you'd have to add extra punctuation if you had a hash of arrays that
you wanted to copy.

%newhash = %{ dclone(\%oldhash) };

How do I define methods for every class/object?

(contributed by Ben Morrow)

You can use the UNIVERSAL
class (see UNIVERSAL). However, please
be very careful to consider the consequences of doing this: adding
methods to every object is very likely to have unintended
consequences. If possible, it would be better to have all your object
inherit from some common base class, or to use an object system like
Moose that supports roles.

How do I verify a credit card checksum?

Get the Business::CreditCard
module from CPAN.

How do I pack arrays of doubles or floats for XS code?

The arrays.h/arrays.c code in the PGPLOT
module on CPAN does just this.
If you're doing a lot of float or double processing, consider using
the PDL
module from CPAN instead--it makes number-crunching easy.

AUTHOR AND COPYRIGHT

This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.

Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit. A simple comment in the code giving
credit would be courteous but is not required.